Plant tissues, organs and systems — AQA GCSE Combined Science: Trilogy
Plant tissues, the leaf as an organ, and how water, mineral ions and sugars move through the roots, stem and leaves.
Revision notes
5 short notes, in the order of the specification. Each one in short:
Plant tissues are groups of cells whose structure suits their function. Epidermal tissues cover and protect the plant, palisade mesophyll has many chloroplasts for photosynthesis, and spongy mesophyll has air spaces for gases to diffuse. Xylem transports water and mineral ions, phloem transports dissolved sugars, and meristem tissue produces new cells for growth.
The leaf is a plant organ made of several tissues that work together. The epidermis covers it, with stomata in the lower epidermis. Most photosynthesis happens in the palisade mesophyll, and the spongy mesophyll has air spaces for gases. Xylem and phloem in the veins bring water in and take sugars away, and guard cells control the stomata.
The roots, stem and leaves form a plant organ system for transport. Root hair cells absorb water by osmosis and mineral ions by active transport, helped by a large surface area and many mitochondria. Xylem, made of hollow tubes strengthened by lignin, carries water and mineral ions up the plant. Phloem moves dissolved sugars by translocation.
Transpiration is the loss of water vapour from the leaves. It draws water from the roots up through the xylem in a continuous transpiration stream. Water evaporates from the mesophyll cells and diffuses out through stomata, which are pores mainly on the underside of the leaf. Guard cells open and close the stomata to control gas exchange and water loss.
The rate of transpiration increases with higher temperature, more air movement and greater light intensity, and decreases with higher humidity, because humid air makes the concentration gradient smaller. The rate is measured with a potometer, which records water uptake by a cut shoot, using rate of transpiration = distance moved by bubble ÷ time.
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Quick check questions
Which plant tissue transports water and mineral ions?
Show answer
Xylem.
What is translocation?
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The movement of dissolved sugars through phloem tissue from the leaves to the rest of the plant.
Why does an increase in humidity decrease the rate of transpiration?
Show answer
The concentration gradient of water vapour between the leaf and the air is smaller, so water vapour diffuses out more slowly.
What do guard cells control?
Show answer
The opening and closing of stomata, which controls gas exchange and water loss.
Frequently asked questions
How does water move from roots to leaves?
Water enters root hair cells by osmosis, then moves up through the xylem to the leaves in a continuous flow called the transpiration stream. In the leaf, water evaporates from the surface of the mesophyll cells into the air spaces, and water vapour diffuses out through the stomata. This loss of water vapour from the leaves is transpiration.
How does light intensity affect the rate of transpiration?
Increasing light intensity increases the rate of transpiration, because stomata open wider in the light, so more water vapour can diffuse out of the leaf. In darkness the rate falls. Higher temperature and more air movement also increase the rate of transpiration, while higher humidity decreases it. A potometer can be used to measure these effects.
What is the difference between xylem and phloem?
Xylem transports water and mineral ions from the roots to the stems and leaves in the transpiration stream, and is made of hollow tubes strengthened by lignin. Phloem transports dissolved sugars from the leaves to the rest of the plant by translocation, and is made of tubes of elongated cells with pores in the end walls.
Why do stomata close?
Stomata close when their guard cells lose water. Closing the stomata reduces the loss of water vapour from the leaf, so the plant saves water. The trade-off is that carbon dioxide can no longer diffuse into the leaf, so photosynthesis slows down. When the guard cells take in water, they swell and the stoma opens again.
Written and checked against the AQA GCSE Combined Science (8464) specification · Updated October 2026